1. Technical Field
This invention relates to a watthour meter or a wattmeter with a precise linearity of the conversion of low voltages into frequency within a broad dynamic range.
In the proposed meters a voltage-to-current converter converts the load voltage drop into electric currents, which are conducted to a multiplication-adding circuit.
The foregoing circuit is composed of 2n multiplication circuits connected in cascade and controlled by two non-overlapping switching signals. Each of said circuits is provided with a Hall sensor. These Hall sensors are either placed in two slits of a single ferromagnetic yoke surrounding a conductor or in a slit of either of two ferromagnetic yokes, each yoke surrounding a different conductor, in which conductors the load current is conducted. The meter further comprises a current-frequency converter, in which there are connected in series an integrator, to whose second input a reference current generator with a controlled reversal of the polarity of its outputs is connected, a comparator, and a logic control circuit, while the output of the multiplication-adding circuit is connected to the first input of the afore-mentioned integrator. A digital signal generator generates a clock signal and basic switching signals.
2. Description of the Prior Art
There has been known a watthour meter or a wattmeter described in patent applications DE No. 37,02,344 A1 and YU No. 564/86, in which meter Hall voltage is converted into frequency. The offset voltage influence is reduced by reversing the integrating capacitor.
In the multiplication-adding circuit according to the patent application YU No. 564/86 three multiplication circuits provided with a Hall sensor are cascade connected to each other. The current terminals of the Hall sensor in the second multiplication circuit are connected to the common terminals of the first and of the fourth switch and to the common terminals of the second and of the third switch. The input current is supplied to the common terminals of the first and of the third switch, while the common terminal of the two other switches is connected to the output of the operational amplifier, to whose inverting input the first voltage terminal of the Hall sensor is connected. The second voltage terminal of the sensor is connected to the noninverting input of the operational amplifier of the third multiplication circuit. The noninverting input of the operational amplifier of the first multiplication circuit is grounded, the second voltage terminal of the Hall sensor of the third multiplication circuit, however, represents the output of the multiplication-adding circuit. The first and the second switch, and the third and the fourth switch, respectively, are controlled by the same non-overlapping switching signals.
The output voltage signal of the described multiplication-adding circuit is susceptible to external interferences. In both known meters the action of charge injections taking place in electronic switches is neglected. Further, monolithic technology is not quite appropriate for a meter with reversing integrating capacitor because of considerable parasitic capacitance of said capacitor. In addition, for said meters the problem of the temperature compensation, of the aging, and of the voltage dependence of the Hall sensor is not resolved.